A semiconductor device is manufactured by repeating a process of transferring a pattern formed by a photomask on a wafer by a lithography process and an etching process. In such a manufacturing process, so as to realize an early launch of yield and stable operation of the manufacturing process, it is essential to quickly analyze defects found by in-line wafer inspection and use the analyzed defects to countermeasure thereabout.
In the in-line wafer inspection, it is common to re-detect defects and foreign matters detected by an optical defect inspection device with a review device disposed at a rear stage of the defect inspection device and to observe the re-detected defects and foreign matters in detail. So as to quickly link inspection results to countermeasures against failure, in particular, automatic defect review and classification technology is key which reviews a large number of detection defects at high speed and classifies the large number of detection defects according to cause of generation. Since the defect size affecting the manufacturing yield of semiconductors has also been miniaturized according to the miniaturization of a semiconductor pattern manufacturing process, reviewing with high resolution is becoming difficult with the optical review device. Therefore, scanning electron microscope (SEM) type review device capable of high-speed and high-resolution review can be commercialized. In this device, it is important to acquire an SEM image (hereinafter also referred to as a shadow image) equivalent to a shadow generated when light is applied from the side so as to detect minute foreign matters and irregularities such as scratches.
Hereinafter, regarding an emission direction of secondary electrons (SE) and backscattered electrons (BSE) (hereinafter collectively referred to as secondary particles) generated by scanning a charged particle beam to a sample, a direction from 0 degrees to 90 degrees from a sample surface toward a normal line of the sample is defined as an elevation angle and a rotation angle around the normal line of the sample is defined as an azimuth.
The basic principle for acquiring such a shadow image will be explained. As an example, a case of a foreign matter having a trapezoidal shape whose an upper side is shorter than a lower side is considered. Irregularities of the trapezoidal shape are scanned with electron beams from an upper side of a cross section thereof. In a case where a right side of the irregularities is irradiated with the electron beam, secondary electrons are emitted. At this time, when attention is paid to a low angle component of the elevation angle of the secondary electron, a portion of the secondary electron emitted to the left side is shielded by the irregularities. Therefore, when two left and right detectors are disposed on the foreign matter side to detect secondary electrons, the numbers of secondary electrons detected by the left and right detectors is different from each other. In other words, the electrons emitted from the right side of the foreign matter are detected more frequently by the detector on the right side than the detector on the left side. In this case, a brightness on the right side of the foreign matter in the SEM image generated by the detector on the right side is brighter than the same position in the SEM image generated by the detector on the left side. When the left side of the foreign matter is irradiated with the electron beam, the state opposite to the above description is obtained. Therefore, the SEM image obtained by the detector described above becomes an SEM image in which the right side of the foreign matter is bright in the detector on the right side and an SEM image in which the left side of the foreign matter is bright in the detector on the left side, and thus becomes an image with the enhanced shadow.
On the other hand, a case of a trapezoidal recess whose upper side is longer than the lower side is considered. In a case where the right side of the recess is irradiated with the electron beam, secondary electrons are emitted. At this time, a portion of the secondary electrons emitted to the right side is shielded by a side surface of the recess. As a result, the electrons emitted from the right side of the recess are detected more frequently by the detector on the left side than the detector on the right side. In this case, the brightness on the right side of the recess in the SEM image generated by the detector on the right side is darker than the same position in the SEM image generated by the detector on the left side. The left side of the recess is irradiated with the electron beam, a state opposite to the above description is obtained. Therefore, the SEM image obtained by the detector described above becomes an SEM image in which the right side of the recess is dark in the right side detector and an SEM image in which the left side of the recess is dark in the left side detector.
The shape of a foreign matter or the like from the acquired SEM image can be discriminated using the principle described above. Furthermore, so as to detect shallow irregularities, fine foreign matter, or the like with high sensitivity, it is important that shadow contrast is emphasized by selecting the detected azimuth and the detected elevation angle for electrons emitted from foreign matter or the like and high quality image is acquired. PTL 1 discloses that “a low angle component and a high angle component of an elevation angle viewed from the point of occurrence of secondary electrons are selected and an azimuth component is also selected and detected” by “applying an electric field that accelerates secondary particles adjacent to a wafer so as to suppress secondary particle energy dependency of rotation of the secondary particle”.